Dually frequency modulated altimeter



Patented Feb. 24, `1948 'v "DUALLY FREQUENCY MODULATED ALTIMETER Daniel Blitz, Princeton, N. J., assigner to Radio Corporationof America, a corporation of Delaware Application Juneai, A194i, serial No. 541,935

Claims. (Cl. Z50-1.68)

My invention relates to radio altimeters or other distance measuring systems of the frequency- .modulated type.

vIn systems of this type thedistance measure mentincludes an error that is an inherent characteristic of such systems and which is referred tofasffixed error. As pointed out in Sanders Patents 2,222,586 and 2,222,587, fixed error re'- sultsfrom the fact thatthe beat frequency signal obtained during each frequency-modulation cycle or sweep may contain a whole number of beat cycles plus a fractionof a cycle. The fraction of a cycle may or may notproduce an extra count in thefrequency counter, depending upon the phase relation of the beat frequency signal with respect to the' modulation' sweep. Thus the distance indication may be in" error by a certain amountthat' is `called the fixed error. The aboveementionedpatents describe methods of reducing xederron The present invention describes another method of reducingvfixed` error which, in some circuit designs, may have advantages'over 'previously proposed methods and devicesnff l l An` object of the invention, as indicated above, is to provide an improvedmethod of and means for minimizing fixed error in a radio altimeter orfrada'r system of the frequency-modulated type. "iA further object of the invention is to provide anf improved radio altimeter or radar system of thefrequency-modulated type.

In practicing the present invention the system includes a radio transmitter that is cyclically frequency modulated at a comparatively high frequency rate in such a way that there is a periodic slowshift in the mid-frequency or carrier frequency of the Vfrequency-modulated signal. It should be noted that this does not change t -e bandwidth of the'high frequency sweep.

' Asfa result of the comparatively slow carrier freqii'ency'l shift, the phase of thebeat frequency cycles with respect to the beginning and end of eachfrequency sweep differs for successive frequency sweeps as explained hereinafter. YAs a result there will be no sustained error in the frequency count; the count will vary from the cor rectl'count plus one to the correct count minus one. fBymaking vthis variation faster than the Figures 3, 4 and 5 are graphs that are referred:

to in explaining the invention.

In, the several figures of the drawing, similar parts are indicated by similar reference characters.

Fig. 1 shows the invention applied to a fre quency-modulated altimeter comprising a transmitter oscillator I0 having a frequency controlling tuned circuit I0a to which is connecteda frequency-modulating unit I I.Y The modulating unit II comprises a'variable capacitor element I2 and a driving coil I3. The modulating unit may be of the'type described and claimed in appli.

Y cation Serial No. 471,003, filed January 1, 1943,

time constant ofthe frequency counter or other trating another embodiment of the invention,and

in the name of Sydney V. Perry and entitled Ca pacity modulator unit.

The'modulating unit II is driven by a frequency-modulating current (which may be of sine wave form as shown in Fig. 3) which is supplied through a, transformer I4 from a frequency-` modulating vwave source I6. This current may have a frequency of cycles per second, for example.

In addition to the frequency-modulated current from source I 6, there is applied to the modulating unit I3 through a transformer I'I a comparatively low frequency current such as shown in Fig. 4 which is supplied from a source I8 for the purpose of shifting the mean frequency or carrier frequency of the frequency-modulated wave. The sum of the two currents applied to the modulating unit I I is illustrated in Fig. 5.

The resulting cyclically frequency-modulated wave with a slowly varying mean frequency is radiated from an antenna 2| toward the earth or other Wave reflecting surface, and the reflected wave is picked up by a receiving antenna 22 and supplied to a detector or mixer 23. There is also supplied to the detector 23 directly from the oscillator I0 over conductors 24 a frequency-modulated wave that is the same as that radiated from the antenna 2l.

As is well known in the art, the two waves combine in the detector 23 to produce a signal having a beat frequency that is a function of the distance to the wave reflecting surface. Because of the low frequency modulation produced by the current source I3, the phase of the beat frequency signal varies at this low frequency, assuming the received wave is being reflected from some-fixed distance. The beat frequency signal is vamplified by an audio frequency amplifier 25 and supplied to a suitable frequency measuring device such as a frequency counter 21. Preferably, a frequency counter of the general type described in the Sanders patent and Schwartz et al. Patents Nos. 2,228,367 and 2,137,859, respectively, is employed. It should be understood that instead of a frequency counter a suitable` frequency selective circuit such as a frequency discriminator and a milliammeter or voltmeter connected thereto may be employed. Whatever the type of frequency indicator, it should have a long enough time constant or suflicient damping or other suitable characteristic to average out signal variations occurring at the rate of the mean frequency variation produced by the current source I8. In the case of a frequency counter, for example, its time constant should be greater than the period of the signal supplied Vfrom the source I3.

It will be understood that the slow rate variation in phase of the beat frequency signal, assuming reflection from a fixed distance, results from the fact that the phase of this signal depends upon the relative phase of the directly transmitted wave and the reiiected wave at the detector 23, this relative phase changing with the change in frequency of the transmitted wave. The cyclic variation in phase of the beat frequency signal means that at the end of each successive fast frequency modulation sweep the beat frequency signal will end at a different point in the frac-I tional cycle whereby the plus one and minus one errorsV occur cyclically and can be averaged out by the frequency counter or other frequency 'indicator.

While the xed error reducing signal from the source I8 has been illustrated in Fig. fl as a triangular wave signal, it should be understood that the signal need not be of this particular wave form and may, for example, be a saw-tooth wave signal. Likewise, the frequency-modulating wave from the source le may be of any desired wave form lsuch as triangular-'or sawtooth, for example. By employing the above described method and system, V'any fixedV error in the distance indication is substantially reduced or eliminated.

Fig. 2 shows an altimeter or distance measuring system similar to that shown in Fig. 1 but modulating said transmitter at a certain rate to produce a frequency-modulated wave, means for simultaneously shifting the mid-frequency of the frequency-modulated wave at a periodic rate that is low compared with said cyclic rate, means for transmitting said frequency-modulated wave to a wave reflecting surface, means for receiving the reflected Wave, means for mixing the received wave with a portion of the modulated wave transmitted directly from the transmitter to produce a beat frequency signal, a frequency counter, and means for supplying said beat frequency signal to said frequency counter whereby a distance indication is obtained, said frequency counter having a time constant or damping period that is longer than the period of said periodic midfrequency shift.

3. In a distance measuring device of the character described, a source of oscillatory currents, means for varying the frequency of said currents, means for varying continuously the mean frequency `of said currents, means for applying the thus Varied currents to an 'antenna to radiate waves from the point from'which distance is to be measured towards a wave reflecting medium,

' means for receiving at said point the ree'cted which differs therefrom in the means for modulating the transmitter escalar-,0r lo. in Fig. 2, the

transmitter oscillator I0 is modulated by means of a vacuum tube reactance circuit 3l, which is well knownv in the art, instead of by means of a mechanically variable capacitor unit. The frequency-modulating wave and the fixed error correcting wave from the source I6 and I8, respectively, are added in the input circuit of a vacuum tube amplifier 32 and applied through a transformer 33 to the reactance tube modulator 3l. The operation of this system is the Same as de-y scribed for the system of Fig. 1.

- I claimv as my invention:

1.,A` distance measuring system'comprising a radio transmitter, means for cyclically frequency modulating said transmitter at a certain rate to produce a frequency-modulated wave, means for simultaneously shifting the mid-frequency of the frequency-modulated wave at a periodic rate that is low 'compared with said'cyciic rate, means for transmitting said frequency-'modulated wave to a wave reflecting surface, means for receiving the reflected wave, means for mixing the received wave with a portion of the modulated wave'transmit'tedl directly from the transmitter to produce a beat frequency signal, a frequency-indicating means, and means for applying said beat frequency signal to said last means whereby a distance indication is obtained, said frequency-indicating means having a'time constant o-r dampingperiod that is longer than the period of said periodic mid-frequency shift.

2. A' distance measuring system comprising a radio'transmittenmeans for cyclically frequency waves and the directly transmitted waves identical with the radiated waves, Vmeans for mixing said waves to produce beat frequency waves, and means fordemodulating said beat frequency waves.

4. In a distance measuring device of the character described, a source of oscillatory currents, means for varyingthe frequency of said currents, means for varying continuously at a comparatively lowffrequency rate the mean frequency of said currents, means for applying the thus varied currents to an antenna to radiate waves from the point from which distance is to be measured towards a wave reflecting medium, means for re-I ceiving at said point the refiected waves and the directly transmitted waves Videntical with the radiated waves, means for mixing said waves to produce beat frequency waves, means for def modulating said beat frequency waves, and a frequency indicating device having a time constant longer than the period of said low frequency variations to which the demodulated waves are applied.

5. A distance measuring system including means for generating waves, means for varying at a certain rate the frequency of said waves about their mean'frequency, means for also varying at a comparatively low frequency rate said mean frequency, means forradiating said waves toward a wave reflecting surface the distance of which is to be determined, means for combining the reiiected waves with waves transmitted direct from the wave generator which are the same as the radiated waves to produce a beat frequency wave, means for demodulating said beat frequency wave, and an indicating device to which said demodulated wave is applied, said device having the characteristic such that it will average signal variations occurring at said comparatively low frequency rate.

DANIEL BLITZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 

